In many areas it is extremely desirable to be able to precisely measure a small displacement of an object relative to some reference position. One area in which this technique is particularly applicable is in determining the relative movement of parts of large engineering structures, for example, bridges and their supports. It is often the case that the bulk of the measurement apparatus cannot be mounted on a surface which can be guaranteed to be fixed with respect to any of the moving parts of interest.
A second area of application is in the determination of the bending of a gun barrel, and in particular the gun barrel of a tank, in order to ensure highly accurate firing of the gun. Bending of a gun barrel can be caused by a number of factors including thermal effects resulting from firing of the gun and/or weather conditions, backlash in the gun mounting following each firing, and vibration due to tank motion. Whilst it is possible to reduce the effects of barrel bending by physically stabilising the barrel, it is not possible to completely eliminate the problem. Efforts have therefore been made to provide systems for determining the extent of barrel bending so that the degree of bending can be compensated for when the gun is being aimed.
There is described in GB 1,587,714 apparatus for correcting sighting errors in a tank gun barrel arising from barrel bending. The system comprises a light source and an adjacent detector, both fixed to the breech end of the gun barrel, or to the tank turret, and a mirror fixed at or near the muzzle end of the gun barrel. A light beam from the light source is directed onto the mirror which reflects the light beam back to the light detector. Any angular displacement of the barrel muzzle relative to the breech end of the barrel causes the returning light beam to be moved across, or off, the light detector. The extent of any angular displacement can therefore be estimated by monitoring the output of the light detector. Other systems are known which project a collimated beam of light from the source to the muzzle mirror and thence back to the detector.
A problem with systems such as that described in GB 1,587,714, and similar systems commonly known as muzzle reference systems (MRS), is that beam deflection can occur due to factors other than displacement of the barrel muzzle. For example, movement of optical components in the transmitting or receiving optical systems can cause such beam deflection. In addition, non-linearity in the light detector itself, or in other components of the detection circuitry, can erroneously indicate barrel displacement. These errors are inevitably translated into misalignment of the gun barrel when the barrel is being aimed. Significant targeting errors can arise from bending of the gun barrel by even a few tens of micro-radians and the known muzzle reference systems are not capable of resolving bending measurements with this degree of accuracy.
It is an object of the present invention to overcome or at least mitigate certain of the disadvantages of known apparatus and methods for determining relative object displacement.
In particular, it is an object of the present invention to provide an automatic muzzle reference sensor (AMRS) system for measuring the angular displacement of a gun barrel muzzle with respect to the breech end of the barrel whilst substantially eliminating errors resulting from transmitting and receiving optics and circuitry.